1. Field of the Invention
The present invention relates to a magnetic compass for determining azimuth by detecting the geomagnetism.
2. Description of the Related Art
Conventionally, there is a technique of determining azimuth by horizontally placing a magnetic sensor with a two-dimensional direction sensitivity and detecting the horizontal component of the geomagnetism.
However, this technique faces significant errors when the magnetic sensor is placed at an inclination.
In view of this, in order to allow accurate azimuth to be obtained even when the sensor is placed at an arbitrary angle, there is a technique in which both a 3-axis magnetic sensor and an inclination sensor are used. The 3-axis magnetic sensor has three mutually orthogonal sensing directions. The inclination sensor is composed of acceleration sensors or the like, and can detect inclination relative to the horizontal plane. The accurate azimuth is determined using the detected geomagnetic vector and correct it with obtained inclination.
However, the use of the inclination sensor may make it difficult to achieve low cost or low power consumption system. Further, the inclination sensor picks up noise due to vibration or mechanical impact, which may inevitably result in the occurrence of errors.
In view of this, with a view to achieving lower cost and higher accuracy, there has been a demand in recent years for a technique that allows inclination detection to be performed without using an inclination sensor. As a technique that meets this demand, there have been disclosed an azimuth measuring device and an azimuth measuring method in Japanese Unexamined Patent Application Publication No. 2005-61969. This technique is proposed as a method that enables azimuth measurement to be performed using a three-axis magnetic sensor even when the azimuth measuring device assumes an arbitrary attitude (inclination).
According to this method, when the user measures an azimuth while holding the azimuth measuring device in horizontal, geomagnetic components in mutually orthogonal directions are detected by geomagnetism detecting means of the azimuth measuring device. This horizontal state is defined as a first attitude. While keeping the azimuth of the azimuth measuring device, the attitude of the azimuth measuring device different from the first attitude is set. This attitude is defined as the second attitude. The first triaxial output data indicative of geomagnetic components in the first attitude, and the second triaxial output data indicative of geomagnetic components in the second attitude are detected by the geomagnetism detecting means, respectively.
Next, on the basis of the first triaxial output data and the second triaxial output data, the angular difference between the first attitude and the second attitude is calculated by angular difference calculating means. Then, on the basis of the first attitude and the angular difference, the second attitude is determined by inclination calculating means. Lastly, on the basis of the second attitude and the second triaxial output data, the azimuth in the second attitude is calculated by azimuth calculating means.
As described above, to perform azimuth measurement using the above-mentioned azimuth measuring device, it is necessary for the user to perform the following extremely complicated operation. That is, the user holds the azimuth measuring device in horizontal, and while maintaining the azimuth obtained in the first attitude, brings the azimuth measuring device into the second attitude that provides the second attitude. For this reason, the azimuth measuring device proves extremely inconvenient and unpractical for measuring the azimuth of a movable body such as an automobile.